Powered air purifying respirators utilize a powered mechanism, such as a blower, to draw ambient air through air purifying elements to remove contaminants from the ambient air. They are designed for use as respiratory protection against atmospheres with solid and liquid contaminants, gases and/or vapors where the concentrations during entry and use are not immediately dangerous to life or health and the atmosphere contains adequate oxygen to support life.
U.S. Patent Publication No. 2003/0180149, Volumetric Control For Blower Filtered Devices, refers to a control unit which determines a differential pressure between at least two measuring points. The differential pressure is converted into a control signal to vary a fan's output. The at least two measuring points can be arranged in the air flow behind a fan impeller and in front of the consumer, in particular, the breathing hood.
U.S. Pat. No. 4,899,740, Respiratory System For Use With A Hood Or Face Mask, relates to a system having a blower (battery powered, electric motor operated) connected between inlet and outlet plenums. The motor can be manually switched between high and low speed operation to supply high quantities or air upon demand caused by a high respiration rate and otherwise supply lower quantities of air sufficient for low respiration rates to extend battery life between rechargings. A differential air pressure sensing switch is responsively connected for actuation when the difference between air pressure in the outlet plenum and the pressure of ambient atmosphere is less than a pre-selected value. A battery powered audible alarm sounds upon actuation of the switch to alert the user to the approach of potentially dangerously low air pressure in the outlet plenum and consequently in the mask or hood.
U.S. Patent Publication No. 2003/0019494, Method And System Of Calibrating Air Flow In A Respirator, provides for the establishment of control set points in a true calibration protocol through the simple triggering of the microprocessor of a controller. When the trigger is initiated, the microprocessor engages and provides the logic for the calibration cycle. The calibration cycle proceeds until a second trigger terminates the process and establishes the control set point. The calibration sequence of the method relies only on an initiation and termination trigger that is facilitated by components integral to the apparatus.